Working with large images and transform in the VSE
Currently not possible to do that without some image quality loss.

If this is allowed then do the following:

Option 1


Add your image as a strip.
With the strip selected add Effect Strip -> Transform.
In Transform strip properties choose Bilinear for interpolation (or try with the other options to your liking).
Now scale the image. Assuming you are using 1920x1080 for render dimensions - type the expression (with numbers) in Scale - for X - [your_image_width]/1920; for Y - [your_image_height]/1080. This will set upscale factors for the image which will restore its original dimensions and also here the image quality loss occurs.
Now you can pan or zoom in the Transform strip using Position and Scale respectively. For Scale from now on always multiply X and Y with the same scale factor. For example if you want to scale the image down in half multiply the values by 0.5. If you want to scale up twice - multiply both values by 2.


Option 2

You can always import the image in the 3D view as plane. For that you can use the plug-in

Import -> Images as Planes (if not there, enable it in User Preferences)


Make the camera orthographic and orient it so it looks directly towards the image plane.
Set the 3D view to Textured. Scale and move the plane from Camera view to your liking.
In VSEditor add the Scene where the image is as a strip. In Sequencer Image View properties choose Scene Preview/Render to be Textured.
You can open 3D View window and animate the scale and position of the plane. To see the updated plane, Refresh the sequencer or change frames in the timeline.


Option 2 has better image quality than Option 1 but requires you to work in the 3D space.